The present invention relates to a heat-shrinkable multilayer film suitable for use as a heat-shrinkable packaging material and comprising at least three layers including an outer surface layer (a) formed of a thermoplastic resin, an intermediate layer (b) formed of a polyamide resin, and an inner surface layer (c) formed of a heat-sealable resin.
Hitherto, it has been widely practiced as a packaging and processing technique for content materials, inclusive of foods such as raw meat, ham and sausage, principally, and other products, to form a casing, such as a bag or a pouch, from a heat-shrinkable multilayer film, and then fill the casing with such a content material, or to automatically package such a content material per se or a tray loaded with such a content material while simultaneously forming a casing from a heat-shrinkable multilayer film.
As heat-shrinkable films for such use, many films comprising a polyamide resin as a principal resin have been proposed heretofore, inclusive of, e.g., a heat-shrinkable polyamide film using a polyamide resin composition comprising an aromatic copolyamide resin which is a copolymer of aliphatic diamine/isophthalic acid and aliphatic diamine/terephthalic acid, and an aliphatic polyamide resin (Japanese Laid-Open Pat. Appl""n (JP-A) 62-41261); and biaxially stretched laminate films or heat-shrinkable multilayer films including an (outer) surface layer comprising a polyester resin, an intermediate layer comprising a polyamide resin and a heat-sealing layer comprising a polyolefin (JP-A 4-99621 corresponding to U.S. Pat. No. 5,336,549, and JP-A 11-300914 corresponding to WO99/55528). These packaging films have succeeded in acquiring respectively intended properties.
However, diversity of properties are required of a packaging film inclusive of, e.g., (a) to (g) described below.
(a) First of all, a packaging film is required to have a strength sufficient to package a content material and provide a skin having a good appearance and allowing a customer to see the content material beautifully therethrough. In order to improve the appearance, it has been widely practiced to heat-shrink the film after packaging the content material. For this purpose, the film should preferably have as large a heat-shrinkability as possible. In this respect, a polyamide resin-based film can be provided with an excellent strength and a large heat-shrinkability through biaxial stretching, and the above-mentioned heat-shrinkable films all have take advantage of such properties of polyamide resin. However, a single layer of polyamide resin cannot satisfy many other properties required of a packaging film.
(b) For Example, a heat-sealing method is adopted in many cases for hermetically sealing a packaging film after packaging the content material, whereas a polyamide resin does not generally have a good heat-sealability. Accordingly, an inner surface layer comprising a heat-sealable resin is disposed on an inner side of the polyamide resin layer.
(c) In case where a moisture-absorptive polyamide resin layer is used to constitute an outer surface layer, the surface layer absorbs water at the time of shrinking with hot water or atmospheric moisture to cause a lowering in strength of the polyamide resin layer when the packaged product is frozen or refrigerated. Particularly, in the case of freezing, the moisture absorbed in the polyamide resin surface layer is frozen to lower the strength thereof. Accordingly, it is preferred to dispose a layer of thermoplastic resin which is moisture-impermeable or has only a small moisture-permeability on an outer side of the polyamide resin layer.
(d) A polyamide resin has a certain degree of gas-barrier property, but it is preferred to dispose a gas-barrier resin layer in addition to the polyamide resin layer for packaging of a content material of which the degradation is extremely undesirable.
(e) The additional layers shown in (b)-(d) above, when used, all function to reduce the good heat-shrinkability after biaxial stretching of a film composed of a single layer polyamide resin.
(f) A film imparted with a heat-shrinkability through biaxial stretching is liable to cause some degree of shrinkage during a period of storage thereafter until its use for packaging or when heated to a temperature of at most 50xc2x0 C., thus causing wrinkles or size change. This is liable not only to impair the appearance of the product film but also to cause a trouble in an automatic packaging step. For removing these difficulties, a heat treatment (for relaxation or heat-setting) of holding the film after biaxial stretching at a temperature of 60-80xc2x0 C. is performed, as desired. This heat treatment however also functions to reduce the heat-shrinkability of the film after biaxial stretching.
(g) As a preferred mode of biaxial stretching treatment of a polyamide resin film, an inflation process has been practiced. In the inflation process, only a certain range of inflation gas pressure is commercially feasible and practically used. As a result, in the case of forming various diameters of inflation bubbles, a larger deformation stress is applied for a small diameter bubble (e.g., in lay-flat width of below 400 mm, particularly below 250 mm) and a smaller deformation stress is applied for a larger diameter bubble, respectively to the film section. Accordingly, it is not easy to provide a laminate film structure capable of exhibiting stable inflation performances (capability of forming a stable inflation bubble and acquiring a high heat-shrinkability owing to a biaxial stretching effect of the inflation) for a wide range of bubbles (i.e., a wide range of lay-flat widths) receiving different biaxial stretching stresses.
Accordingly, a principal object of the present invention is to provide a polyamide resin-based heat-shrinkable multilayer film capable of satisfying the above-mentioned properties (a)-(g) at high levels, and particularly capable of exhibiting a highest level of heat-shrinkability even after heat-setting.
According to our study, it has been found possible to achieve the above-mentioned object by a combination of an intermediate layer (b) comprising a very limited composition of polyamide resin, an outer surface layer (a) comprising a specific composition of thermoplastic resin, an inner surface layer (c) comprising a specific composition of sealable resin, and appropriate biaxial stretching and heat-treating steps.
Based on the above-knowledge, the present invention provides a heat-shrinkable multilayer film, comprising: at least three layers including an outer surface layer (a) formed of a thermoplastic resin, an intermediate layer (b) formed of a polyamide resin, and an inner surface layer (c) formed of a heat-sealable resin; wherein
the thermoplastic resin forming the outer surface layer (a) comprises a polyester resin or a polyolefin resin,
the polyamide resin forming the intermediate layer (b) comprises a mixture of 85-60 wt. % of an aliphatic polyamide resin, and 15-40 wt. % of an aromatic copolyamide resin which is a copolymer of aliphatic diamine/isophthalic acid and aliphatic diamine/terephthalic acid,
the heat-sealable resin forming the inner surface layer (c) essentially comprises a copolymer of ethylene and an xcex1-olefin having a density below 0.915, and
the heat-shrinkable multilayer film has been biaxially stretched and then heat-treated to have hot-water shrinkabilities at 80xc2x0 C. of at least 30% in each of longitudinal/transverse directions and at least 35% in at least one of longitudinal/transverse directions.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawing.